[ruby-core:63031] [CommonRuby - Feature #8259] Atomic attributes accessors

Issue #8259 has been updated by Eric Wong.


 [e-mail] wrote:
 > Adding "atomic" operations to built in `Array`s, `Hash`es etc. is a very bad idea
 > because that would force that *all* of the methods on `Array`s and `Hash`es be
 > concurrency friendly and would result in irreparable performance problems for
 > truly concurrent Ruby VMs (and this again would put an upper limit on Ruby
 > performance).
 
 I do atomic operations all the time in C on arbitrary addresses.
 Lazy, non-atomic accesses run without speed penalty if I don't need
 up-to-date data.
 
 The uncommon case of Array/Hash shrinkage would require RCU or similar
 (epoch-based reclamation).  But there's no penalty for reads or in-place
 modifications other than the cost of the atomic and required memory
 barriers.
 
 Ruby swap/cas (in Ruby) should probably raise on non-existent Array/Hash
 elements.

----------------------------------------
Feature #8259: Atomic attributes accessors
https://bugs.ruby-lang.org/issues/8259#change-47114

* Author: Yura Sokolov
* Status: Open
* Priority: Normal
* Assignee: 
* Category: 
* Target version: Ruby 2.1.0
----------------------------------------
=begin
Motivated by this gist ((<URL:https://gist.github.com/jstorimer/5298581>)) and atomic gem

I propose Class.attr_atomic which will add methods for atomic swap and CAS:

  class MyNode
    attr_accessor :item
    attr_atomic :successor

    def initialize(item, successor)
      @item = item
      @successor = successor
    end
  end
  node = MyNode.new(i, other_node)

  # attr_atomic ensures at least #{attr} reader method exists. May be, it should
  # be sure it does volatile access.
  node.successor

  # #{attr}_cas(old_value, new_value) do CAS: atomic compare and swap
  if node.successor_cas(other_node, new_node)
    print "there were no interleaving with other threads"
  end

  # #{attr}_swap atomically swaps value and returns old value.
  # It ensures that no other thread interleaves getting old value and setting
  # new one by cas (or other primitive if exists, like in Java 8)
  node.successor_swap(new_node)

It will be very simple for MRI cause of GIL, and it will use atomic primitives for
other implementations.

Note: both (({#{attr}_swap})) and (({#{attr}_cas})) should raise an error if instance variable were not explicitly set before.

Example for nonblocking queue: ((<URL:https://gist.github.com/funny-falcon/5370416>))

Something similar should be proposed for Structs. May be override same method as (({Struct.attr_atomic}))

Open question for reader:
should (({attr_atomic :my_attr})) ensure that #my_attr reader method exists?
Should it guarantee that (({#my_attr})) provides 'volatile' access?
May be, (({attr_reader :my_attr})) already ought to provide 'volatile' semantic?
May be, semantic of (({@my_attr})) should have volatile semantic (i doubt for that)?
=end




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